No large population of unbound or wide-orbit Jupiter-mass planets

Abstract

Planet formation theories predict that some planets may be ejected from their parent systems as result of dynamical interactions and other processes. Unbound planets can also be formed through gravitational collapse, in a way similar to that in which stars form. A handful of free-floating planetary-mass objects have been discovered by infrared surveys of young stellar clusters and star-forming regions as well as wide-field surveys, but these studies are incomplete for objects below five Jupiter masses. Gravitational microlensing is the only method capable of exploring the entire population of free-floating planets down to Mars-mass objects, because the microlensing signal does not depend on the brightness of the lensing object. A characteristic timescale of microlensing events depends on the mass of the lens: the less massive the lens, the shorter the microlensing event. A previous analysis of 474 microlensing events found an excess of ten very short events (1-2 days)-more than known stellar populations would suggest-indicating the existence of a large population of unbound or wide-orbit Jupiter-mass planets (reported to be almost twice as common as main-sequence stars). These results, however, do not match predictions of planet-formation theories and surveys of young clusters. Here we analyse a sample of microlensing events six times larger than that of ref. 11 discovered during the years 2010-15. Although our survey has very high sensitivity (detection efficiency) to short-timescale (1-2 days) microlensing events, we found no excess of events with timescales in this range, with a 95 per cent upper limit on the frequency of Jupiter-mass free-floating or wide-orbit planets of 0.25 planets per main-sequence star. We detected a few possible ultrashort-timescale events (with timescales of less than half a day), which may indicate the existence of Earth-mass and super-Earth-mass free-floating planets, as predicted by planet-formation theories.

Innocent Bystanders: Orbital Dynamics of Exomoons During Planet–Planet Scattering

“…Replacing n moon,eject with the fraction of free-floating giant planets that carry moons (∼ 0.08) and using the same estimate for other parameters as above yields an occurence rate of O(10 −3 -10 −2 ) per star. If using the observed frequency of free-floating planets per star (0.25) (Mróz et al 2017) and multiplying it with the fraction of moon-bearing free-floating planets from the simulations (∼ 0.08), the occurence of moonbearing free-floating planets per star is O(10 −2 ). Both approaches yields probabilities that are not insignificant.…”

“…−0.8 , and Mróz et al (2017) found N f f Nstar = 0.25 for Jupiter-mass free-floaters. The equation below adopts a smilar method to Veras & Raymond (2012) to estimate the number of free-floating moons:…”

A Neptune-mass Free-floating Planet Candidate Discovered by Microlensing Surveys

“…Current microlensing surveys are capable of detecting free-floating planets down to Earth-mass objects. To this day, however, all reported free-floating planet candidates were based on the very short timescale of an event (t E 2 days) and lacked direct measurements of the angular Einstein ring size (Sumi et al 2011;Mróz et al 2017). OGLE-2016-BLG-1540 is the first case for which we procured such a measurement, owing to the fortuitous fact that the source was a giant.…”

“…OGLE-2016-BLG-1540 is the first case for which we procured such a measurement, owing to the fortuitous fact that the source was a giant. If the source were a dwarf (with at least ten times smaller angular radius), as in the case of ultrashort candidate events detected by Mróz et al (2017), the finite-source effect would be significantly weaker. We simulated the OGLE light curve and found that the finite-source model would be preferred only by ∆χ 2 = 1.6 over the point-lens model.…”

OGLE-2017-BLG-1130: The First Binary Gravitational Microlens Detected from Spitzer Only

“…In event OGLE-2014-BLG-0124 ), the planetary signal was detected independently from Spitzer, and if the trajectories had been slightly different, the planetary signal could have been detected by Spitzer and missed from the ground. In addition, such binaries may affect the inferred timescale distribution from observed events (Sumi et al 2013;Wyrzykowski et al 2015;Mróz et al 2017;Wegg et al 2017), because in event OGLE-2017-BLG-1130 the timescale fitted from ground data is about 10 days shorter than the real case. Therefore, the role that Spitzer plays in microlensing observations is more than functioning as a parallax satellite, and it will produce more results of scientific interest in the future.…”